The present invention relates to multilayer optical bodies containing at least two different materials that form a reflective interface for at least one polarization of light. The present invention also relates to optical films (including multilayer optical films) that include a birefringent copolymer of polyethylene naphthalate.
Polymeric films are used in a wide variety of applications. One particular use of polymeric films is in mirrors and polarizers which reflect light of a given polarization and wavelength range. Such reflective films are used, for example, in conjunction with backlights in liquid crystal displays to enhance brightness and reduce glare of the display. A polarizing film can be placed between the user and the backlight to direct the light towards the user and to polarize the light; thereby reducing the glare. A mirror film can be placed behind the backlight to reflect light towards the user; thereby enhancing brightness. Another use of polarizing films is in articles, such as sunglasses, to reduce light intensity and glare.
One class of polymers that is useful in creating polarizer or mirror films is polyesters, described in, for example, U.S. Pat. Nos. 5,825,543 and 5,867,316 and PCT Publications WO 99/36262 and WO 97/32226, incorporated herein by reference. One example of a polyester-based polarizer includes a stack of polyester layers of differing composition. One configuration of this stack of layers includes a first set of birefringent layers and a second set of layers with an isotropic index of refraction. The second set of layers alternates with the birefringent layers to form a series of interfaces for reflecting light.
The optical properties of a given polyester are typically determined, at least in part, by the monomer materials utilized in the preparation of the polyester. A polyester can be prepared by reaction of one or more different carboxylate monomers (e.g., compounds with two or more carboxylic acid or ester functional groups) with one or more different glycol monomers (e.g., compounds with two or more hydroxy functional groups).
Generally, the present invention relates to optical bodies and their manufacture, as well as the use of the optical bodies in optical devices, such as polarizers and mirrors. One embodiment is a multilayer optical film that includes birefringent first optical layers and second optical layers interleaved with the first optical layers. Each first optical layer is formed using a copolymer of polyethylene naphthalate with less than 70 mol % of the carboxylate subunits being naphthalate subunits. The second optical layers have a lower in-plane birefringence than the first optical layers for 632.8 nm light. The copolymer of the first optical layers can be a random or block copolymer. The invention is also directed to methods of making and using the optical films and devices containing the optical films.
Another embodiment is a multilayer optical film that includes birefringent first optical layers and second optical layers interleaved with the first optical layers. Each first optical layer is formed using a polymer (e.g., a copolymer of polyethylene naphthalate) that is capable of yielding an in-plane birefringence of at least about 0.16 for 632.8 nm light after orienting the polymer at a temperature no more than about 5xc2x0 C. above the glass transition temperature of the copolymer or, if desired, no greater than the glass transition temperature of the copolymer. The second optical layers have a lower in-plane birefringence than the first optical layers for 632.8 nm light. Preferably, the polymer is capable of yielding an in-plane birefringence of at least 0.18 for 632.8 nm light, more preferably, at least 0.19, after orienting the polymer at a temperature no more than about 5xc2x0 C. above the glass transition temperature of the copolymer or, if desired, no greater than the glass transition temperature of the copolymer. The invention is also directed to methods of making and using the optical films and devices containing the optical films.
Yet another embodiment is a multilayer optical film that includes birefringent first optical layers and second optical layers interleaved with the first optical layers. Each first optical layer is formed using a copolymer of polyethylene naphthalate having no more than about 20% crystallinity, as determined by differential scanning calorimetry. The second optical layers have a lower in-plane birefringence than the first optical layers for 632.8 nm light. The invention is also directed to methods of making and using the optical films and devices containing the optical films.
A further embodiment is a method of making a multilayer optical film. A stack of optical layers is formed. The stack of optical layers includes first optical layers, made using a copolymer of polyethylene naphthalate, and second optical layers forming optical interfaces with the first optical layers. The first optical layers are then oriented at a temperature of no more than 5xc2x0 C. above the glass transition temperature of the copolymer and, if desired, no greater than the glass transition temperature of the copolymer to give the first optical layers an in-plane birefringence of at least about 0.16, preferably, at least 0.18, and more preferably, at least 0.19.
Another embodiment is a multilayer optical film that includes birefringent first optical layers and second optical layers interleaved with the first optical layers. Each first optical layer is formed using a polymer (e.g., a copolymer of polyethylene naphthalate) that is capable of yielding an optical efficacy of at least about 0.10. The second optical layers have a lower in-plane birefringence than the first optical layers for 632.8 nm light. Optical efficacy is defined as (nzxe2x88x92ny)2/(nxxe2x88x92ny)2, where nx, ny, and nz are the indices of refraction of the optical layer.
The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures and the detailed description which follow exemplify several embodiments.